1. Field of the Invention
This invention relates to fuel cell coolant compositions with good corrosion inhibiting properties and low electrical conductivities, and to their use to inhibit corrosion in fuel cells.
2. Description of Related Art
Fuel cells are electrochemical cells in which the chemical energy stored in a fuel source is converted to electrical energy by controlled oxidation of the fuel. In principle a fuel cell operates like a battery. However, unlike a battery, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes sandwiched around an electrolyte. For example, in a proton exchange membrane fuel cell, gaseous hydrogen (H2) is supplied to the anode, usually a porous metal plate which acts as a catalyst for the oxidation reaction. An oxygen (O2) source, which can be simply air, is supplied to the cathode, which is also usually a porous metal plate. The electrodes (i.e., the anode and cathode) are, as noted, separated by an electrolyte, an ionically conductive material through which ions can flow from the anode to the cathode. In the case of a proton exchange membrane fuel cell, the electrolyte is split by a thin solid polymer sheet that is permeable to protons (i.e. hydrogen ions, H+). At the anode, a hydrogen molecule dissociates to release two electrons and two protons i.e.H2→2H++2e.The protons and the electrons produced by this reaction travel from the anode, where they were made, through the membrane to the cathode, where they are used up in the reduction of oxygen:½O2+2H++2e→H2O.The electrons can do useful electrical work on their way from the anode to the cathode if a load is placed across the electrodes to complete the circuit, and produce energy. Thus, the products of proton exchange membrane fuel cells are electrical energy and water. The relatively low output of pollutant products compared to the output of pollutant products from combustion processes make fuel cells attractive alternatives in applications including environmentally friendly automobiles and power plants. Notably, proton exchange membrane or “PEM” fuel cells are now found in the propulsion systems of most prototype fuel cell cars and buses.
The electrical potential (voltage) of a fuel cell is determined by the electrochemical potentials of the fuels and oxidants used in the fuel cell, and the total current available from a fuel cell is determined by the total surface area of the electrodes. Many single electrochemical cells can be stacked together in series in order to generate greater voltages, and the resulting greater number of electrodes and consequent greater electrode surface areas also allow greater currents than a single cell. The potential difference between the cathode and anode on the first and last cells (the positive and negative ends of the stack, respectively) is roughly equal to the number of cells in the stack multiplied by the voltage of each cell. The stack can consist of hundreds of individual PEM fuel cells assembled together to produce enough electricity from the chemical reaction of oxygen and hydrogen to run a car or bus.
In a fuel cell stack, the individual fuel cells are separated by plates made of an electrically conductive material such as carbon, and these separator plates are electrically connected. Heat generated by the fuel cell stack can be removed by flowing water or other fluids through channels in the separator plates between the cells. These often ionically conductive coolant fluids, such as glycol, are directed through a conduit that is manifolded to pass through separator plates in parallel, where they are collected at the other side of the cells. The heat of a cell stack may cause the fluids to vaporize, and the vapor may be condensed elsewhere in the fuel cell system. Alternatively, the heat absorbed by the fluids may simply be radiated out to the surrounding environment, and the fluids recirculated through the stack.
The potential difference between the positive and negative ends of the fuel cell stack tends to cause a shunt current to flow through the cooling fluid, thus reducing the voltage of the fuel cell. In addition to the deleterious loss of voltage, shunt currents create the additional problem of causing the separator plate nearest the positive end of a fuel cell stack to corrode with time. Thus, there is a need in the art for fuel cell coolants that have superior electrical resistance in order to prevent shunt currents from reducing the fuel cell potential and to reduce corrosion of fuel cell separator plates while maintaining good heat conductivity so that they are useful as fuel cell coolants.
Fuel cells have been studied as power sources for automobiles. However, in order for this application to be practical, the fuel cells must be able to start even in freezing weather. Thus there is also a need in the art for fuel cell coolants that have low freezing point temperatures.